The Eph subfamily of receptor tyrosine kinases mediates multiple aspects of neural development, including long-range axonal pathfinding, synaptogenesis, and synaptic plasticity. In addition, a disruption of Eph function has been linked to neurological disorders such as autism and Alzheimer's disease. In preliminary studies we identified a ligand-independent EphB signaling mechanism whereby the guanine nucleotide exchange factor Ephexin5 associates with the cytoplasmic domain of EphBs and suppresses excitatory synapse development through activation of the small GTPase RhoA. This brake on excitatory synapse development is relieved by binding of EphBs to their ephrinB ligands, which triggers rapid Ephexin5 tyrosine phosphorylation, ubiquitination, and degradation. Knockout studies in mice indicate that EphB-Ephexin5 signaling functions as a synaptogenesis checkpoint so that excitatory synapses form at the right time and place and in correct numbers during brain development. Ephexin5 loss-of-function mutations have been detected in several cases of idiopathic infantile epilepsy. We have also identified Ephexin5 as a novel neuronal substrate of the E3 ubiquitin ligase Ube3a. Loss-of-function mutations in UBE3A give rise to Angelman syndrome (AS), a neurodevelopmental disorder characterized by motor dysfunction, severe mental retardation, speech impairment, and seizures; and our preliminary findings suggest that elevated levels of Ephexin5 contribute to at least some aspects of the neurological and cognitive dysfunction associated with AS. In this proposal we address specific gaps in our understanding of the importance of the EphB-Ephexin5 complex in nervous system development and disease. Our specific aims are: (1) to investigate the mechanisms by which EphB- Ephexin5 signaling controls synapse development, (2) to investigate the contribution of Ephexin5 mutations to human infantile epilepsy, and (3) to determine the contribution of elevated Ephexin5 levels to phenotypic defects observed in a mouse model of AS. It is our hope that the proposed experiments will advance our understanding of the molecular mechanisms controlling synapse development and ultimately provide new opportunities for the development of therapeutic strategies to combat neurodevelopmental disorders such as infantile epilepsy and autism.